The present invention relates to the oxidation protection of carbon/carbon composites that are composed of a carbon matrix reinforced with carbon or graphite fibers, and more particularly, to the oxidation protection system achieved by impregnation of the composite with ceramic precursors dissolved in a supercritical fluid.
Impregnation of low density composite bodies has been typically accomplished by immersing a ceramic part in a bath of a chemical solution, slurry, or melt, often under vacuum to assist the removal of trapped air. Multiple immersions are commonly necessary to achieve sufficient penetration and densification. With these conventional impregnation techniques, complete penetration of the internal porosity has been extremely difficult or impossible due to the sealing-off of passageways to the interior of the body which occurs during the impregnation process. Moreover, sufficient densification/impregnation may also be difficult due to the surface tension and high viscosities of some impregnants or large slurry particle sizes relative to the available openings in the body.
Supercritical fluids are gases and liquids at conditions above their respective thermodynamic critical points which exhibit: high pressure-dependent solvent power for many substances of normally low solubility; near ambient temperature processing capability, low viscosity and high diffusivity; and the absence of surface tension. For any particular supercritical fluid, at sufficiently high pressure, the isobaric solubility of a material increases as a function of temperature. At a given temperature (above the critical temperature of the fluid) a decrease in pressure reduces the solubility of the dissolved material in the fluid.
Supercritical fluid phenomena have been investigated primarily for purposes of extraction. Recent applications have included such processes as regeneration of activated carbon, separation of alcohol from water, and extraction of oils and pharmacological compounds.
Supercritical fluids, for example, have been used for the recovery of certain materials from foodstuffs and other starting materials. U.S. Pat. No. 3,806,619 (Zosel), for example, discloses the use of supercritical carbon dioxide for recovery of caffeine. U.S. Pat. No. 4,104,409 (Vitzhum et al) describes the removal of certain resins from hops using supercritical carbon dioxide and other compounds. U.S. Pat. No. 4,167,589 (Vitzhum et al) shows the impregnation of dearomatized, decaffeinated tea using supercritical fluids such as carbon dioxide. U.S. Pat. No. 4,354,922 (Derbyshire et al) shows a dense gas solvent, in a supercritical fluid state above its critical temperature and pressure, used to extract heavy hydrocarbon oil constituents. The Derbyshire et al '922 patent teaches lowering of pressure (while maintaining the temperature above the critical temperature), or raising the temperature, to precipitate out the dissolved hydrocarbon constituents. It has thus been demonstrated that supercritical fluids are applicable for extracting normally insoluble materials and removing them from a base material. Vitzhum et al '589 also teaches that supercritical carbon dioxide can absorb certain aromatic constituents of tea, and upon subsequent dissociation can redeposit these aromatics in the tea. U.S. Pat. No. 4,241,112 (Kostandov et al) discloses the successive deposition of an organometallic catalyst on the surface of a solid filler. The deposition of the second component of the catalyst is gas or liquid phase deposition, and a simultaneous polymerization of olefins on a first deposited catalyst component is carried out at temperatures which in some cases fall within the supercritical regime.
U.S. Pat. No. 4,552,786 to Berneburg et al which is incorporated here by reference, teaches the use of supercritical fluids to deposit a ceramic precursor in the void spaces of a ceramic, silicon nitride, silicon carbide or aluminum borosilicate host material.
None of the references have applied supercritical fluid technology to the impregnation of carbon/carbon composites with oxidation resistant ceramic precursors. Partially as a result of their inherent porosity, carbon/carbon composites are not as oxidation resistant as is desired. Until now, external coatings, which are prone to separation (spalling) from the surface, and particulate dispersions of non-oxide ceramics in the carbon matrix, were the only usable forms of oxidation protection currently available.